1. Field of the Invention
The present invention relates to cleanroom construction and particularly to cleanroom ceilings and frames therefor, including the mounting of ceiling panels and cleanroom air filters on supporting beams or cross members and the suspension of lighting fixtures, wire conduits, or other hardware from the cross members between the filters. More particularly, the present invention relates to a ceiling structure which eliminates the vortex formations formed below the cross members and improves the airflow in a cleanroom for more uniform, unidirectional laminar flow therein by directing airflow through the ceiling structure.
2. State of the Art
The Electronics industry has imposed ever more rigorous purity requirements on cleanrooms where sensitive components are manufactured. Several years ago, class 100 cleanrooms (averaging no more than 100 particles of 0.5 microns diameter in one cubic foot of controlled air space) were acceptable, while requirements today often exceed class 1 based on 0.1 micron diameter particles. See, for example, prior art patents disclosing cleanroom structures include U.S. Pat. Nos. 3,158,457; 3,638,404; 4,667,579 and 4,693,173. Cleanroom ceilings, walls, and floors must therefore be constructed in such a manner as to minimize convection and eddy currents, dead air spots, and other areas which tend to collect dust and other particulate matter and/or disturb the uniform airflow in the cleanroom. Because of the moving air within the cleanroom, both convection currents and dead spots tend to form small, swirling pockets of air near the ceiling, referred to herein generally as vortices. These pockets capture and accumulate particulate material.
Generally, cleanrooms require filtered airflow and/or the uniform flow of filtered air from the ceiling to and through the floor. The airflow originates from blowers situated between the cleanroom and the filters. The air from the blowers is forced through cleanroom air filters overlaying a portion of, or the entire ceiling of, the cleanroom and travels downwardly from the ceiling through the cleanroom, exiting through the floor. The ceiling filters are generally mounted on a grid of ceiling support beams or cross members, the bottom surfaces of which may be in close proximity with the bottom surfaces of the filters.
Although the diffusion screen assists in developing laminar flow of the air
exiting the filters, the desired uniform flow pattern is interrupted immediately below the ceiling surface by vortex regions and dead air space beneath the cross members. These vortex regions form because of low pressure arising below the cross members in the absence of airflow, causing a dead space where particulate material can accumulate. The size and geometry of the vortex will vary, depending upon the width of the cross member and the velocity of airflow emanating from the adjacent filters.
The uniform flow pattern is also disturbed by light fixtures and other attachments which are suspended from the cross members. For example, the high intensity lighting systems used in cleanrooms generally comprise extended linear arrays of fluorescent light tubes traversing the width and/or length of the cleanroom ceiling. The bottom surfaces of the support beams generally are used for the attachment of these light fixtures and are also used to attach mounting apparatuses for supporting modular walls and similar hardware. These attachments extend into the cleanroom from the ceiling plane formed by the ceiling filters and beams, creating convection currents and collection points for particulate matter which impair the purity of the cleanroom.
In the past, efforts to place these light fixtures within the cross members have been frustrated by the need for minimizing the vortex and dead air space present under the width of the cross member. Placement of the light fixture within the cross member would necessarily increase this width in order to provide adequate volume to fully contain the fixture. Accordingly, a typical practice continues the use of tear drop configuration of lights which suspends the fixture below the cross member.
Nevertheless, the increasingly stringent requirements for minimal contamination within the cleanroom requires modification of cleanroom ceiling structure to a flush mounted system. Referring to drawing FIG. 2, the Brod McClung-Pace Co. has introduced a flush ceiling system which depicts a widened cross member 10 having an enlarged channel 11 for receiving a light fixture 12. A gel track 13 supports cleanroom filters 14 in a position located above the channel 11. A screen member 20 is attached below the filter 14 in a manner which is represented to have reduced the vortex region 16 under the cross member to within 2 inches of the flush surface 17. Typically, a vortex and any non-uniform velocity area will extend 3 to 4 times the grid width. The actual depth of the vortex associated with the Pace system is suggested to be only one-half the distance between adjacent filters.
Another point of concern is that no suitable arrangement of cleanroom ceiling fixture attachments has yet been developed which maximizes uniformity of noncontaminated airflow while at the same time offering compatibility with conventional cleanroom ceiling structure such as conventional cleanroom filters with a lower mounting flange or knife edge positioned at the base of the filter while minimizing or eliminating vortex formations and dead air spaces. The Pace "under slung" structure requires use of a special filter 14 whose mounting flange 15 is positioned at an upper portion 19 of the filter. Such compatibility with conventional low mounting flange or knife edge structure is not only important from a viewpoint of economy in construction, but the conventional filter with lower mounting flange offers a known advantage of better sealing which is known and trusted within the industry. Any use of a non-HEPA standard filter only results in industry resistance to the arrangement as well as such as arrangement being viewed as inferior or undesirable by the industry. Accordingly, the use of conventional cleanroom filters avoids such problems.
Neither has such a system been developed for general use with flush lighting systems in ceilings of non-cleanroom environments, e.g., Lonseth, U.S. Pat. No. 4,175,281, Lipscomb, U.S. Pat. No. 3,173,616.
In U.S. patent application Ser. No. 07/973,067, filed Nov. 6, 1992, the assignee of the present invention discloses and illustrates a ceiling structure within a cleanroom which includes an array of standard cleanroom filters supported in grid support structure approximately flush with an exposed surface of the cleanroom filters to the cleanroom interior. Means are provided for flushing a vortex space immediately below the grid support structure and between the respective openings of the grid structure with a channeled air stream to remove particulate contaminant.
One prior art embodiment of the '067 patent application which uses a standard cleanroom filter with a knife edge seal at the bottom is illustrated in drawing FIG. 3. This embodiment provides a cross member 33 having a top wall 35 and opposing side walls 36 and 37. These side walls extend down to a gel track 38.
This gel track 38 is accordingly coupled to the cross member by being integrally formed as a single extrusion with the side walls 36 and 37 near a lowest interior perimeter of each of the openings 32 in the grid support structure.
The function of the gel track 38 is to provide a trough for containment of a sealing gel which receives a peripheral flange or knife edge 44 which is coupled to and supports the cleanroom filter material 45. This peripheral flange 44 includes a sealing edge 46 which is suspended within the gel track in near proximity with the ceiling level 50.
The inside, side walls 36a and 37a form a vertical extension of the respective side walls 36 and 37 and provide a mounting base for integral attachment of the base side 39, 40 and remaining inclined side walls 41 and 42.
This inclined side wall structure which provides means for flushing a vortex space or region (referred to hereafter as vortex) 51.
The specific purpose of the inclined side wall structure is to provide means for generating a stream of airflow 52 toward this vortex 51 which effectively sweeps particulate matter into a desired laminar flow with the remaining airflow generated through the grid openings.
A screen is mounted with respect to the gel track and filter support structure 66 by means of a second peripheral flange 67, forming a "Z" configuration.
Another embodiment of the '067 patent application is illustrated in drawing FIG. 4 and shows a perimeter wall structure 73 which provides an angled Z configuration. This angled Z is formed at its base by the perforated screen 71 and couples to a first perimeter wall 72 of the screen which is substantially parallel with the inclined wall 74 of the gel track. The space between the first perimeter wall 72 and inclined wall 74 forms the flow channel. The remaining angled Z structure includes a section of screen wall 75 comprising an upper inclination cooperates with the peripheral flange structure 72 and 75, and openings 78 to direct airflow toward the vortex space.
While these embodiments provide an improved airflow distribution and reduce the size of any disturbance or vortex formation below the sealed lighting area of the ceiling grid, it is desirable to eliminate the disturbance area or vortex below the lighting area to provide laminar, unidirectional airflow throughout the cleanroom.